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11 December 2010

BOBGRAM7 issued Sun 12 Dec 2010

WEATHERGRAMYOTREPSIssued 12 December 2010Bob McDavitt's ideas for sailing around the South Pacific.

Disclaimer: Weather is a mix of pattern and chaos; these ideas are fromthe patterned world of weather maps, so please fine-tune to your place.Dates are in UTC unless otherwise stated. Oceanic Index

La NINA: Still strong. Average atmospheric SOI over past 30 days is 2.10(a rise of 0.41 in the past week). Oceanic Nino Index for Sep-Oct-Nov is-1.4 and steady.

TROPICSSouth Pacific Convergence Zone SPCZ returned to normal last week, drapedacross the Coral Sea and Vanuatu with build ups over Fiji and Samoa. Andthere is scattered activity along 15S to French Polynesia FP.

A aging low centre near 25S 170E (to SE of New Caledonia) has acirculation that is drawing SPCZ activity southwards across Fiji andTonga. This centre is expected to fade as it wanders southwardstowards Norfolk Island, and SPCZ activity should continue to hug 15Sthis week.

There is a Madden Julian Oscillation MJO of enhanced convection makingits way across the Timor Sea this week and into the Coral sea between 15and 20 Dec. This should activate the SPCZ. Normally it increases therisk of cyclone formation, but this MJO is likely to lack a zone ofequatorial westerlies so may not have much impact in the Coral Sea.

Some models are picking that a zone of weak equatorial westerlies maytrigger a tropical low or two to form off NW Australia. The latest GFSmodel is picking one to form in Gulf of Carpentaria between Tuesday 14and Friday 17 Dec and for this to then move SW then S inland towardsAlice as a wet Monsoon Low. In any event an Australian conveyor belt ofupward motion is being set up in the heat trough from North Australia toQueensland, sometimes reaching Tasman Sea and then onto New Zealand, andthis NW flow in the upper air is likely to last for the next fortnight.

SUBTROPICAL RIDGE: STR. The STR was knocked north last week but a weak Polar outbreak (polarindex went negative). Southern lows took the opportunity to expandnorthwards and one passed by close to NZ - It is now near 45S 165W andit seems destined to deepen as it wanders north getting as far north as40S 150W by Wed 15 Dec then exiting to the southeast. Associated Highis making its way east along 30S. Another high is forming just east ofNew Zealand tonight Sun 12 Dec. This should wander along 45S on Mon 13Dec the slide around south-side of that low, but leave a cell near 30Sso that enhanced trade winds are expected all week from FP to Tonga.

Another High is expected to cross NZ Wed /Thu 15/16 Dec and then blossomeast of NZ and slowly shift north to 35S by Tue 21 Dec.

TASMAN SEA/ NZ AREASome more southern lows are likely to get near NZ this week. One isexpected to pass by south of Macquarie Island tonight Sun 12 Dec and anassociated front is expected to swipe by NZ on Monday/Tue 13/14 Dec,followed by a showery southerly change.

A small low may form in the Australian conveyor belt off Queenslandaround Wed 15 dec and bring some welcome rain to northern NZ in a NEflow around Thursday 16/Friday 17 Dec. This may start a trend.

Then another southern low should pass south of Tasmania around Sat 18Dec as an accompanying low may form off Sydney, feeding from theactivity in the Australian conveyor belt. These lows may dance aroundeach other around Sun 19 Dec, whizzing close by Southland and blasting astrong northerly flow across most of NZ. Avoid. Its frontal band isthen expected to cross NZ on Monday-Tuesday 20-21 Dec.

Keep an eye on that conveyor belt as it is likely to continue producingLows in eastern Aussie seaboard to NZ area from 23 to 27 Dec, and thismay have an impact of the Sydney-Hobart.The terms used are more fully explained in the METSERVICE Yacht Pack. More info at http://weathergram.blogspot.com Feedback to bob.mcdavitt@metservice.com